1. Field of the Invention
The present invention relates to processes for coating substrates with a solventborne clearcoat composition over a waterborne basecoat composition.
2. Discussion of Background Material
In prior art processes for making coated objects, particularly automotive coatings, manufacturers have utilized coating systems which involve the use of both basecoats and clearcoats. Traditionally, both the basecoats and clearcoats were applied as solventborne compositions. However, more recently manufacturers have become increasingly concerned about environmental impact due to the emission of organic solvents into the atmosphere during the application and curing of solventborne coating compositions. As a result of this concern and environmental regulations associated therewith, it is becoming increasingly desirable to utilize waterborne coating compositions.
It has been found that if a waterborne basecoat composition containing free amine is applied to a substrate, followed by the application of a traditional solventborne clearcoat composition comprising a monomeric melamine as a crosslinking resin, upon simultaneous curing of both layers, the resulting cured film appears "wrinkled". This appearance is undesirable and lacks commercial value. It has been found that clearcoat compositions utilizing polymeric melamine do not exhibit this wrinkling problem. As a result, the current commercially available clearcoat compositions utilize polymeric melamine as the crosslinking entity, as opposed to monomeric melamine. However, the use of monomeric melamine could permit formulating a higher solids composition and could result in improved physical properties of the resulting coating, if only the wrinkling problem could be solved.
The inventor of the present invention has unexpectedly discovered that the above described advantages of high solids concentration and improved physical properties can be achieved by utilizing a solventborne monomeric melamine clearcoat composition which comprises a high level of acid catalyst. Such a composition has been unexpectedly found to achieve the advantages of improved physical film properties as well as increased solids content in the clearcoat composition, while avoiding entirely the wrinkling problem described above.
However, compositions comprising a "high level of acid catalyst", in combination with monomeric melamine, are not new. Such compositions have previously been utilized in the "refinish" industry, i.e. in the repainting of automotive body panels after damage to the finish. However, such repair processes do not utilize waterborne basecoat compositions in combination with the high level of acid catalyst, monomeric melamine-containing clearcoat compositions. That is, such use of a high level of acid catalyst in monomeric melamine compositions has traditionally been limited to use in conjunction with solventborne basecoats. Furthermore, such repair processes are carried out at low temperature (i.e. temperatures of from about 160.degree. F. to about 210.degree. F.).
In contrast, the process of the present invention utilizes a high level of acid catalyst in a monomeric melamine composition applied over a layer of an uncured waterborne basecoat, not to mention use over a waterborne basecoat which further comprise a free amine.
In the art of producing automotive topcoats, it is desirable to utilize less organic solvent in coating compositions which are "organic solvent based". Organic solvents serve to disperse (and dissolve) polymers, oligomers, monomers, and other organic components in the composition, in order that the viscosity of the mixture is low enough that the dispersion can be sprayed, etc. However, the presence of organic solvents in the coating composition ultimately results in the release of the organic solvent into the atmosphere, because the solvent is released in a curing step carried out at elevated temperatures. One way of reducing the amount of organic solvent released into the atmosphere is to utilize low viscosity components in the mixture, so that less organic solvent is needed to achieve the desired viscosity. One example of such a low viscosity crosslinking agent is monomeric melamine.
As was described above, the use of monomeric melamine as a crosslinking agent has been found to be unsatisfactory in the event that it is present in a solventborne clearcoat composition which is applied over a waterborne basecoat composition comprising an amine, followed by simultaneous curing of both the resulting uncured waterborne basecoat layer, as well as the uncured solventborne clearcoat layer coating composition (for the clearcoat). The result is a cured coating which exhibits a most unsatisfactory "wrinkled" appearance. Such wrinkled coatings have no substantial commercial value.
However, the present invention provides a process by which an uncured layer of a solventborne clearcoat composition comprising a monomeric melamine can be applied directly over an uncured layer of a waterborne basecoat composition comprising an amine, with both of the layers thereafter being simultaneously cured, to produce a substantially wrinkle-free, cured coating. A first advantage of this process is that less organic solvent need be used (hence less organic solvent is released into the environment). A second advantage of this process is that a higher concentration of solids can be present in the solventborne clearcoat composition, which reduces the volume of composition required. A third advantage of this process is that the resulting cured coating exhibits improved physical properties.
The process of the present invention produces the above-described advantages through the use of a relatively high concentration of an acid catalyst in the solventborne clearcoat composition. This high level of acid catalyst ensures an adequate degree of catalysis for the crosslinking of the organic polymer in the clearcoat composition. That is, the high level of the acid catalyst ensures adequate crosslinking during the curing step, regardless of the presence of the free amine, which has a retarding effect upon the crosslinking of the organic polymer. Thus the acid catalyst is present in a quantity sufficient to both: (1) provide the necessary catalysis for the crosslinking reaction required for the curing of the clearcoat composition, and (2) prevent an undesired retardation of the crosslinking of the clearcoat composition.